Bitumen, in this case, refers to the naturally occurring heavy oil deposits such as the Canadian bitumens found in Cold Lake and Athabasca. Bitumen is a very complex mixture of chemicals and typically contains hydrocarbons, heteroatoms, metals and carbon chains in excess of 2,000 carbon atoms. A variety of technologies are used to upgrade heavy oils, including bitumens. Such technologies include thermal conversion, or coking, that involves using heat to break the long heavy hydrocarbon molecules. Thermal conversion includes such processes as delayed coking and fluid coking. Delayed coking is a process wherein a heavy oil stream is heated to about 932° F. (500° C.) then pumped into one side of a double-sided coker where it cracks into various products ranging from solid coke to vapor products. Fluid coking is similar to delayed coking except it is a continuous process. In fluid coking, the heavy oil stream is heated to about 932° F. (500° C.), but instead of pumping the heavy oil to a coker it is sprayed in a fine mist around the entire height and circumference of the coker. The heavy oil cracks into a vapor and the resulting coke is in the form of a powder-like form, which can be drained from the bottom of the coker.
Another technology used to upgrade heavy oils is catalytic conversion which is used to crack larger molecules into smaller, refineable hydrocarbons in the presence of a cracking catalyst. High-pressure hydrogen is often used in catalytic conversion. While catalytic conversion is more expensive than thermal conversion, it produces a higher yield of upgraded value product.
Distillation is also used for upgrading heavy oils including bitumens wherein the heavy oil is distilled in a distillation tower into a variety of products that boil at different temperatures. The lightest hydrocarbons with the lowest boiling points travel as a vapor to the top of the tower. Heavier and denser hydrocarbons with higher boiling points collect as liquids lower in the tower.
While the above mentioned technologies are useful for converting a portion of heavy oils to lighter and more valuable products, such technologies are not particularly useful for reducing the sulfur content of such feedstocks. One important technology that has been used to reduce the sulfur content (as well as nitrogen and trace metal content) from such feedstocks is hydrotreating. In hydrotreating, or hydrodesulfurization, the heavy oil is contacted with hydrogen and a suitable desulfurization catalyst at elevated pressures and temperatures. The process typically requires the use of hydrogen pressures ranging preferably from about 700 to about 2,500 psig and temperatures ranging from about 650° F. (343° C.) to about 800° F. (426° C.), depending on the nature of the feedstock to be desulfurized and the amount of sulfur required to be removed.
Hydrotreating is efficient in the case of distillate oil feedstocks but less efficient when used with heavier feedstocks such as bitumens and residua. This is due to several factors. First, most of the sulfur in such feedstocks is contained in high molecular weight molecules, and it is difficult for them to diffuse through the catalyst pores to the catalyst surface. Furthermore, once at the surface, it is difficult for the sulfur atoms contained in these high molecular weight molecules to sufficiently contact the catalyst surface. Additionally, such feedstocks may contain large amounts of asphaltenes that tend to form coke deposits on the catalyst surface under the process conditions, thereby leading to deactivation of the catalyst. Moreover, high boiling organometallic compounds present in such oil feedstocks decompose and deposit metals on the catalyst surface thereby diminishing the catalyst life time. Severe operating conditions cause appreciable cracking of high boiling oils thereby producing olefinic fragments which, themselves, consume hydrogen, thereby lowering the process efficiency and increasing costs.
Alternate desulfurization processes that have been employed in the past utilizing alkali metal dispersions, such as sodium, as desulfurization agents. One example of such a process involves contacting a hydrocarbon fraction with a sodium dispersion. The sodium reacts with the sulfur to form dispersed sodium sulfide (Na2S). However, such a process has not proven to be attractive, particularly for treatment of high boiling, high sulfur content feedstocks due to: (a) the high cost of sodium, (b) problems related to removal of sodium sulfide formed in the process, (c) the impracticability of regenerating sodium from the sodium sulfide, (d) the relatively low desulfurization efficiency due, in part, to the formation, of substantial amounts of organo-sodium salts, (e) a tendency to form increased concentrations of high molecular weight polymeric components (asphaltenes), and (f) the failure to adequately remove metal contaminants (iron, nickel, vanadium) from the feed as is observed in the competitive catalytic hydrodesulfurization process.
While various attempts have been made to mitigate some of the above-mentioned problems, low desulfurization efficiency has still remained an unsolved problem. It has been speculated that the low efficiency is due in part to the formation of organo-sodium compounds formed either by reaction of the sodium with acidic hydrocarbons, addition of sodium to certain reactive olefins or as products obtained when sodium cleaves certain of the organic ethers, sulfides and amines contained in the oil. Formation of these organo-sodium compounds, which are desulfurization inactive materials, effectively removes a portion of the sodium that otherwise would be available for the desulfurization reaction. Sodium in excess of the theoretical amount for desulfurization must therefore be added to compensate for organo-sodium compound formation. Moreover, a hydrocarbon insoluble sludge which forms in the course of the sodium-treating reaction (apparently comprised primarily of organo-sodium compounds), makes the reaction mixture extremely viscous and hence impairs mixing and heat transfer performance in the reactor.
Some work has been done to develop electrochemical processes to desulfurize crudes and heavy oils, such as bitumen. Electrochemical processes, such as that taught in U.S. Pat. No. 6,877,556 require the use of reagents such as solvents, electrolytes, or both. Use of such expensive reagents adds to the complexity of those processes since their recovery from the bitumen is required for economic reasons such processes are not commercially practiced.
Therefore, there remains a need in the art for improved process technology-capable of effectively and economically removing sulfur from heavy petroleum feedstock.